Influence of stress simulation parameters on the fracture strength of all-ceramic fixed-partial dentures

Summary

Objectives

This in vitro study tested the influence of diverse stress simulation parameters on the fracture strength of all-ceramic three-unit fixed partial dentures (FPDs).

Methods

All-ceramic FPDs made of Empress 2 (Ivoclar-Vivadent, FL) were exposed to thermal cycling and mechanical loading (TCML) with varying loading parameters such as chewing force (amount, frequency), thermal loading, lateral jaw motion, abutment material, artificial periodontium or antagonistic denture. To investigate the influence of the abutment material, human teeth, polymer abutments and alloy abutments were used. Two different TCML devices with pneumatic or weight loading were compared. FPDs without aging were used as a control.

Results and Significance

Combined thermal and mechanical loading significantly reduced the FPD fracture resistance from 1832N to 410N. Duplication of chewing frequency, phase load increase or additional lateral movement did not effect the results. Increasing chewing force, artificial periodontium, and antagonist or abutment material reduced the fracture resistance of the tested FPDs. Different devices with weight or pneumatic loading had no significant influence on the loading capacity of the FPDs. Artificial aging should be performed combining thermal cycling with mechanical loading. Simulation of the artificial periodontium, human antagonists and abutments should be included to achieve a significant aging.

Introduction

In vitro studies help develop dental materials, but only the experience of the dental scientist allows evaluation of the results to estimate the principal applicability of the material under clinical conditions. In fact, only a clinical trial implies a final warranty of a new material. Nevertheless, in vitro tests, which simulate the clinical conditions allow a first efficient estimation of the material in the shape of a dental restoration. Indeed it is necessary to correlate the in-vitro testing with the clinical experience-but well performed clinical trials are rare.

Various simulation devices and their modifications were introduced, e.g. by DeLong and Douglas [1], [2], Krejci et al. [3], [4] and others [5], [6], [7], [8]. Partly these machines are commercially (EGO, G; EnduraTEC, USA; Willytech, G; SDE, USA) available. The devices try to approach the clinical situation by simulating oral parameters such as chewing force (amount, frequency), thermal loading, moisture, lateral jaw motion, human abutments, periodontium or antagonistic denture. Beyond this, some devices vary the shape of the applied force (sinusoidal, rectangular) or simulate an increasing force (staircase) for Weibull statistics. Special devices were constructed to simulate wear on specimens (Three-body wear ACTA [9], Pin-on block/disc, OHSU), and some ‘artificial mouths’ allow to imitate attrition and abrasion wear in combination with food bolus on dental restorations.

The mastication devices simulate mastication loading (ML), some devices in combination with simultaneous thermal cycling (TC). The force is applied with weight, compressed air or via electric motor and varies between 12 and 70 N. The thermal loading is generated with distilled water of one temperature (37 °C). When temperature changes were required, the testing chambers were floated with cold and hot water (e.g. 5 and 55 °C), or more progressive, the water is continuously circulated to guarantee constant temperatures. The generally short exposure to the moisture environment (less than 10 days) allows only a minor diffusion and therefore marginally contributes to the aging process.

Unfortunately, there are no generally accepted TCML parameters and standardization is out of reach. It is known, that variations of loading frequency [10], [11] or the type of abutment material [12], [13] contribute to the aging process, but it is unclear what consequences were to be expected using different adjusted simulation parameters. Beyond this, the question arises, whether different TCML devices show comparable aging results. The aim of this investigation was to compare the influence of different simulation parameters on the properties of dental restorations. All ceramic FPDs were investigated as an example of dental restorations for their high susceptibility by cracking, torque and bending [10], [14]. The FPDs were aged with different adjustments or TCML devices and their resistance to occlusal loading before and after aging was determined. The results were compared to available clinical data.